High-vacuum device



March 20, 1956 J. R. ROEHRIG 2,739,233

HIGH-VACUUM DEVICE Filed Feb. 25, 1 52 Power Supply l I 1 {To VacuumSysfem AMPLIFIER FIG. I

INVENTOR. JGHNATHAN R. ROEHRIG Y 0M w ATTORNEY United States HIGH-VACUUMDEVICE Application February 23, 1952, Serial No. 272,999

8 Claims. (Cl. 324-33) This invention relates to high vacuum and moreparticularly to pressure gauges for measuring absolute pressure fromatmospheric down to pressures on the order of 10" mm. Hg abs. Thepresent invention is primarily concerned with improvements in gauges ofthe type described in U. S. Patent No. 2,497,213, issued February 14,1950 to I. R. 0. Downing.

The Downing gauge comprises a radioactive source whose rate of emissionof ionizing agents is substantially constant and is substantiallyindependent of temperature and electrical field therearound. Thisradioactive material is positioned to radiate ionizing agents into aspace between two electrodes so as to ionize gas molecules which arewithin an ionization chamber holding the two electrodes.

In a preferred embodiment of the Downing gauge the source of ionizingagents is an alpha particle emitter, such as radium, or a beta particleemitter, such as strontium 90. The invention will be more particularlydescribed in connection with the use of such charged particles asionizing agents, although the general principles are clearly broadlyapplicable to other types of ionizing agents. by collecting ionsproduced therein and amplifying and indicating the ion current socollected. The ionization current bears a direct relationship to thecomposition and absolute pressure of the gas within the gauge. Over asubstantial range of pressures this relationship is that of simpleproportionality. Departures from this proportional or linearrelationship between gas pressure and ionization current are encounteredat still higher pres sures. This departure results from the combinedeffect of two principal causes. First, the number of ion pairs (i. e.,one positive ion and one electron) produced by an ionizing particle intraversing a given mass of gas varies significantly as the ionizingparticle approaches the end of its range. Secondly, at high pressuresthe atent The ionization within the chamber is measured concentrationsof positive ions and electrons, produced by the ionizing particle,become sufiiciently great so that a significantly large portion of thesepositive ions and electrons recombine before reaching the collectingelectrodes. It is, of course, necessary that the positive ion and acorresponding electron reach their respective electrodes in order toappear as ionization current. Ions recombined in the gas do notcontribute to this current.

In designing an ionization chamber for some specific range of operatingpressures as, for example, from about 10 mm. to atmospheric pressure,the above considerations may be applied as follows: It is desired thatthe ion current, which is collected by the electrodes, be taken onlyfrom those ion pairs which are produced by ionizing particles which havetraveled considerably less than their mean residual range. By meanresidual range is meant the mean range (in units of length) of theionizing particles after they enter the gas whose pressure is to bemeasured. This range depends somewhat on the composition of the gas and,naturally, is inversely pro portional to the pressure of the gas.

For example, in

2,739,283 Patented Mar. 20, 1956 the case of alpha particles emanatingfrom radium, this mean residual range is the mean range, at the pressureof the gas, of the alpha particles after they have passed through thefilm of rhodium which is preferably used to confine the decay products(e. g., radon) of radium. Another consideration is that the distancebetween the two electrodes in the ionization chamber should be as smallas possible so as to discourage recombination of the ion pairs. Inpractice, the above two considerations are amply provided for if themean eifective limits of the ionization chamber are maintained less thanthe mean residual range of the ionizing particles at the maximumpressure to be measured. Since the electrodes are within this ionizationchamber, the distance between electrodes is considerably less than thisparticular mean residual range.

Since, at absolute pressure on the order of 1000 mm., the mean residualrange of alpha particles, for example, is relatively short (of the orderof one inch), a very small effective ionization chamber is required forlinear readings in the atmospheric pressure range. On the other hand,the measurement of very low pressures cannot be readily achieved with avery small ionization chamber, since the ratio of dark current topositive ion currents is excessively large. This dark current" is due,at least in part, to alpha particle bombardment of the positive ioncollector electrode. At low pressures, the positive ion currentgenerated in the gas is very small, since it is a direct function of thepressure and the limits of the ionization chamber, as expressed (forair) by the equation I=5.25 l0- LNP, wherein I is ionization current inamperes, P is the pressure to be measured in millimeters, N representsthe useful activity of the radioactive material in alpha particles persecond directed into the ionization chamber, and L represents the meandistance in centimeters between the source of alpha particles and theeffective limits of the ionization chamber.

While it is possible to use a dual ionization chamber with two separatesources of ionizing agent activity, and suitable switching means forobtaining linear readings over a wide pressure range, this dual systemhas the disadvantage that two separate sources of ionizing agent (e. g.,radium) must be employed. Since the radioactive materials utilized inthese Downing gauges constitute an appreciable amount of the total costof such gauges, the use of two radium plaques, for example, in a singlegauge is disadvantageous from the standpoint of economics and health.

- Accordingly, a principal object of the present invention is to providean improved pressure gauge of the Downing type which has a linearresponse over a wide range of pressures from atmospheric pressures topressures on the order of 10 mm. Hg abs.

Another object of the present invention is to provide such a gauge whichemploys only a single source of ionizing agent activity.

Still another object of the invention is to provide a gauge of the abovetype which includes a small ionization chamber within a larger chamber,this smaller ionization chamber being electrically isolated from thelarge chamber to prevent migration of positive ions from one chamber tothe other.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

The invention accordingly comprises the apparatus possessing theconstruction, combination of elements and arrangement of parts which areexemplified in the following detailed disclosure, and the scope of theapplication of whichwill be indicated in the claims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawings wherein:

Fig. l is a diagrammatic, schematic diagram showing one ptefetxcdem dimnt mihe; invention; and

F sa lnlar d f a menta y view otaw tien. of s 1- I [In the pre ninvention t ere pr vided a-fitst cles: o d n n a e ti ly lar e ion zaion ch mber. an a. first. col cto lec ro h this arge. hamb r- M aa arinc ud d. or m ain ng a pot ntial differenc be wee the t o. le trod s st atfinos ti i n a ollec ed y One ofv he lec e n t e. p t cles (i ns. anectro s). r collected; y the. o er of the el c rod s- In a preferred emdi th nven ion the e res o i izin a nt t t isa alpha particl emitte s.such a r m; w ich will. r ate positive ions as a i ect fiu r i n Qt p esure.- Th fir l ct o (wh rl; define t e. on za on. hamb r) is. ma ntaineda a potential V1, and a second electrode is maintained at a pot nt a t ePot ial v2 beinsn eati e. re ative to V1 n de hat the e n electrod may cllect he Posit v nn W thout ntent, t lim t the nvention i l e Par smhr yde r bed in conne t on. w th th use of such an alpha emitter as thesource of ionizing agent activity and with the various electrodesarranged sothat the collection of positive ions is used to measurepressure.

The gauge also includes a third electrode, which delines a secondionization chamber, this second, chamber being relatively small withrespect to the, first ionization chamber. This second ionization chamberis also provided with a collector electrode, the fourth electrode in thegauge, The source of ionizing: agent activity (e. g., radium) ispositionedso that the alphaparticles emitted therefrom traverse both thelarge and small ionization chambers. In a preferred embodiment ofi theinvention the radium is positionedwithin, or; closely adjacent, thesmall ionization chamber and its alpha particlestraverse the smallionization chamber before entering the large ionization chamber. Thealpha particles then pass through the electrode defining'the smallionization charm her, and then traverse the larger ionization chamber.

With this arrangement of elements, the collection of positive ions isusedto' indicate the pressure within the gauge. The electrode whichdefines the smaller ionization chamber is permeable to alpha particlesandis maintained at a potential V3 (preferably equal= to V1) topreventmigrationof positive ions from onechambcnto the other. Inthisconnectionit isparticularly important thatmigration ofpositive ionsfrom the larger to the smaller chamber be avoided when the smallerchamber is being used to measure relatively high -pressures.

Referring now to Figs. 1 and 2, therein shown one preferred embodimentof theinvent-ion which-is particularlyqadapted' for use with a radiumplaque'which serves as a source of ionizing agent.- activity. Inthesedigures; the gauge comprises a housing lowwhi'ch is sufficiently ruggedtowithstand; evacuation. This housing-or casing preferably serves asanelectrode (having the. potential V1) and defines therewithin anionization: chamber: 12. Within the chamber. 12 is a. collectorelectrode 14; arranged to be maintained at a potential -vm more negativethan V1 so as, to collect the positive. ions. A. secondionizationFchamber-defining electrode-:16; is positioned within.- thechamber 12, this second electrode, 16. being at a potential V2. whichpreferably. equals. Y1. Within theionization chamber 20; defined bv the562036: electrode 16 there is provided another positiverioni. collectorlse: w e-.,1: ic smai a n da'atam en iatJYn-prefsm v qual; to. 1 vAsllust an he-electrode 1. pr ierably compr es rli ra tv f ho pr haped.rusmh rs which ser e. a a gri Or p ta s as n u rmitti g passagetof highe ergx alph partic e t st-vi e tor n positive.

pair of conducting strips 23, these conducting strips 23 being securedto the housing 10. This radium plaque 22 is preferably of the typedescribed in the above mentioned Downing patent and is arranged so thatit is in equilibrium with its immediate decay products.

The two positive-ion collector electrodes 14 and 18 are preferablysupported by insulators 24 and 26, respectively, and are alsoconncctedto electrical leads 25 and 27, respectively.

As seen in Fig. 1, the electrical circuit for measuring thepositive-ioncurrents collected by the two electrodes 14 and 18 comprisesan amplifier, generally indicated at 30., This. amplifier includes a:meter 32- preferably calibrated directly in increments of pressure, andis arranged to give a direct indication of the absolute pressure withinthe chamber 12. The amplifier 30 is preferably of the type described byN. P. Moody in Rev. Sci. Instruments 22, No.. 4., 236 (19,51).. In.conjunction with. this amplifier circuit, there is preferably provided arange-shifting mechanism for reading low pressures, in.- termediate.pressures and high pressures. This rangeshifting mechanism preferablyincludes a resistance net- WQlZk comprising resistances vR1,,R2', R-3and R4 and a plurality of thermally operated relays RY-1,,RY-2 andRY-ii. In one preferred. embodiment, the various resistauces. have thefollowing values:

R-I=1,,O00: megohms R' :2=.l'0,000. megohrns has: 100,000 megohms. RA:1,000 megohms The relays RY-l, RY-2 and RY-3 are connected to pressurerange selector buttons B1,, B2, and B respectively, so that theappropriate range can be selected by the operator. A zero set selectorbutton B is also provided, this selector button being arranged to close.relays RY-1' and" RY-3, although the circuitry for accomplishing this isnot shown,

The above resistance network, comprising resistances R-'1, R-2, R-3, andR-4', serves as. a particularly desirable arrangement for selectivelyfeeding, to .a single, amplifier, equivalent voltages corresponding to.the widely .di-fi f'erent ionization currents from the two ioncollectors. At low gas pressures, the ionization current from coll'ector14; alone is used the current. from collector 118 being grounded throughRY -l'. The ionization current from collector T4 is relatively small andthe highresistance R-S serves as the input resistance to the amplifier.This arrangement of input resistors serves for ahundredfold; range ofpressure, say 121 to P2. For still higher pressures, as P; to P3, therelay RY -2 is closed and R-l' serves as the input resistance to theamplifier. By this means, a range of four. decades (i. e., Ba=P1X10isavailable. If the linear range of-the-largt v chamber 12 and itscollector-electrode 14 cover morethan this pressure range of fourdecades, additional lower valuedresistors and associated relays (notshown) can beplaced in parallel with R--1 and, RY-Z across the input tothe amplifier. At still higher gaspressures, as P; to P theionizationcurrent from collector 18 alone is used, the ionization currentcollectedby collector" 14- being grounded by relay R*Y+3; relays RY-l andRY-2being open. The ionization current from collector. 18 isrelatively largeand the relatively low resistance 'R-4 serves as the input resistance;

Since at-the high; pressures above- P4- the ion current collected byelectrode 18' is inherently large, for com veniently constructed sizesofchamber 20, his necessary that; the input: resistance by whichel'ectrode- 181s connected to ground} he not-- too high. This isachieved, as showmby theguse fresi'stor R-4, with-resistorsR-S andRi-Z'pdCtllJji as avoltage divider in a three-resistor circuit I ofsatisfactorily low-netinputresistance pnsappr a hinathegr d 6.-Ashillustrated, -ri artiu1a; v

There are severaljadvantag'es to theaise oftheabove arrangement: Eirst,the number of componentsconnected to the-amplifier input is reduced toamiuiinum, thusp-re serving the extremely high-resistance leakage pathnecessary for proper operation. Second, by the same token, the inputshunt capacitance is kept low, permitting rapid response to pressurevariations. Third, the use of thermally operated relays is madepossible, whereby stray magnetic fields deleterious to the amplifieroperation may be avoided.

The insulators 24 and 26, shown in considerable detail in Fig. 2, arepreferably provided with guard rings 40 and secondary insulators 42 soas to provide a very low effective leakage current for these insulators.These guard rings 49 are preferably connected to an electrical lead 44which is at about the same potential as the potentials V2 and V4 of theelectrical leads 25 and 27. This guard ring 40 thus serves to shield thetwo output leads 25 and 27 from the positive potential on the housing10. The circuit diagram of Fig. 1 also shows a preferred relationship ofthe potentials V1 and V3 to the potential of the nearby conductingobjects considered as ground. As seen, V1 and V3 are preferablyconnected directly to ground to eliminate shock hazard and thepossibility of malfunction from accidental grounding of the chamber 10.As is apparent from Fig. 2 the position of the radium plaque 22 is suchthat alpha particles emanating therefrom pass through the smallionization chamber 20 defined by the grid electrode 16. The alphaparticles pass between the gaps in the electrode 16 and then enter thelarge ionization chamber 12. For measuring high pressures the positiveions created within the small ionization chamber 20 are collected byelectrode 18. The small size of the chamber 120 permits linear responseof the gauge at high pressures due to the fact that the dimensions areless than the mean residual range of the alpha particles, even atpressures on the order of 1,000 mm. Hg abs. In order to maintain thislinearity, the distance between the source of alpha particles and thelimits of the ionization chamber 20 are preferably kept below about0.25' centimeters. The fact that the electrode 16 is at the potential V3substantially prevents the migration of positive ions, generated outsideof the ionziation chamber 20, from entering the ionization chamber 20and thus destroying the linearity of the pressure measurement.

When the pressures become below about mm. Hg abs., the collectorelectrode 14, in the large ionization chamber 12 is used for collectingthe positive ions gen- 'erated therewithin by the alpha particles whichpass through the openings in the electrode 16.

In the operation of the gauge ilustrated in Fig. 1 it is connected to avacuum system by means of a suitable fitting 28. If the vacuum system isat atmospheric pressure, the selector button B3 is pushed so that onlythe positive ion current collected by the collector 18 is fed to theamplifier. When the pressure in the vacuum system has been reduced tobelow about 10 mm. Hg abs, the selector buttons Bl and B2 are actuatedso that now only the positive ion current collected by the collector 14is fed to the amplifier. The fact that both relays RY-1 and RY-2 areclosed, however, permits reading of relatively high ionization currents.When the pressure drops below about 0.10 mm. Hg abs, the selector buttonB2 is deenergized so that very small ionization currents, whichcorrespond to the low pressures, can be read. When it is desired toobtain a zero-set reading, selector button B0 is actuated, this servingto close the relays RY-l and RY-3 and preventing the feeding of anypositive ion currents into the amplifier.

f While the invention has been described particularly with respect to apreferred modification thereof, numerous alternative embodiments may beemployed without departing from the invention. For example, the smallchamber may be positioned outside of the large ionization chamber 12 inan auxiliary housing fastened to housing 10. Additionally, the housing10 may be made longer and narrower, if desired, and the small ionizationchamber may be formed in one end of this long narrow housing.

"6 Equally, it is not essential that the radium plaque 22 be positionedso that it actually defines one side of the small ionzation chamber.However, it is desired that the small ionization chamber and the radiumplaque be so related that the alpha particles emitted from the radiumplaque pass through the small ionization chamber before passing into thelarge ionziation chamber. Additionally, the plaque can separate the twoionization chambers so that radiation emitted from one side of theplaque enters one chamber while radiation from the other side of theplaque enters the other chamber. This arrangement is less desired sincea radium plaque having equal emissivity from both sides is morediflicult to prepare. Equally, the electrode 16 which defines the smallionization chamber may take a number of difierent shapes. It can be awire mesh or any suitable arrangement which will not unduly hinder thepassage of alpha particles therethrough. If desired, electrode 16 can beat a more positive potential than the housing 10. Equally, the housing10 may be a grid-like casing positioned within a second, more rugged,vacuum-tight housing. However, this is not necessary with the particularcircuit shown, and merely adds to the insulating problems. r

While radium has been described as a preferred source of ionizing agentactivity, numerous other materials such as beta ray emitters (e. g.,strontium or gamma ray emitters (e. g., filtered radiation from radium)may be employed. Gamma ray emitters are least desirable since theirionization efficiency is least and their health hazard is greatest. Inthose cases where the ionization agents are other than alpha particles,suitable modifications in the circuitry may be employed. For example,where beta rays are the ionization agents their use may requireditferent input resistors since the ionization produced by beta rays isless than the ionization produced by alpha particles.

While it is preferred that positive ions be collected to indicatepressure, negative ions can be equally collected by suitable reversal ofpolarity in the gauge and by modifying the amplifier.

Since certain changes may be made in the above apparatus withoutdeparting from the scope of the invention herein involved, it isintended that all matter contained in the above description, or shown inthe accompanying drawing, shall be interpreted as illustrative and notin a limiting sense.

What is claimed is:

1. A gas pressure gauge operable at pressures above atmospheric pressurewithout damage thereto and comprising an electrode which defines anionization chamber, an ion collector within the ionization chamber,means for maintaining said ion collector at a potential different thansaid electrode, a second electrode defining a second and smallerionization chamber which communicates with said first ionizationchamber, a second ion collector positioned in said smaller ionizationchamber, means for maintaining a potential diflerence between saidsecond electrode and said second ion collector, said second electrodebeing maintained at a potential which will repulse ions of the typecollected by said second ion collector to prevent migration of said ionsinto said smaller ionization chamber, a single radioactive source whoserate of emission of ionizing agents is substantially constant andsubstantially independent of temperature and electric field therearound,said second electrode being substantially permeable to said ionizingagents, said radioactive source being positioned to radiate ionizingagents into said two ionization chambers, means conductively connectedto said collectors for amplifying current created by the collection ofions at either of said collectors, and means connected to the output ofthe amplifying means for in dicating the magnitude of said amplifiedcurrents and thus the gas pressure within said ionization chambers.

2. A gas pressure gauge operable at pressures above atmospheric pressurewithout damage thereto and comprising an electrode which defines anionization chamber, an ion collector within the ionization chamber,means for maintaining said ion collector at a potential different thansaid electrode, a second electrode defining a second and smallerionization chamber which communicates with said first ionizationchamber, a second ion collector positioned in said smaller ionizationchamber, means for maintaining a potential difference between saidsecond electrode and said second ion collector, said second .elec-.trode being maintained at a potential which will repulse ions of thetype collected by said second ion collector to prevent migration of saidions into. said smaller ionization chamber, a single radioactive sourcewhose rate of emission of ionizing agents is substantially constant andsubstantially independent of temperature andelectric field therearound,said second electrode being substantially permeable to said ionizingagents, said radioactive source being positioned to radiate ionizingagents into said two ionization chambers, the limits of said smallerionization chamber being less than the mean residual range of saidionizing agents at the maximum pressure to be measured, meansconductively connected to said collectors for ampli fying currentcreated by ions collected at either of said collectors, and meansconnected to the output of the amplifying means for indicating themagnitude of said amplified currents and thus the gas pressure withinsaid ionization chambers.

3. A gas pressure gauge. operable at pressures above atmospheric,pressure without damage thereto and comprising an electrode whichdefines an ionization chamber, an ion collector within. the ionizationchamber, means. for maintaining said ion collector at a potential whichis negative with respect to said electrode a second electrode defining asecond. and smaller ionization chamber which communicates with saidfirst ionization chamber, a second ion collector positioned in saidsmaller ionization chamber, means for maintaining a potential differencebetween said second electrode and said second ion collector so that saidsecond ion collector collects positive ions; said second electrode beingmaintained positive with respect; to said first collector so as toprevent migration of positive ions into said smaller ionization chamber,a single radioactive source whose rate of emission ofijonizing agents issubstantially constant and substantially independent of temperature andelectric field therearoupd', said second electrode being substantiallypermeable to said ionizing agents, said radioactive source beingpositioned to radiate ionizing agents into saidtwo ionization cham'bers, means conductively connected to said collectors for amplifyingcurrent created by the positive ions collected at either of saidcollectors, and means connected to the output of the amplifying meansforindicating the magnitude of said amplified currents and thus the gaspressure within said ionizationchambers.

4. A gas pressuregauge operable at pressures above atmospheric pressure.without damage thereto and com: prising an electrode which defines anionization chamber, an ion collector within theionization-chamber, meansfor maintaining said ion collector at a potential which is negative withrespect to said electrode, a second electrode defining a second andsmaller ionization chamber which communicates with saidYfirst-ionizationchamber, a second ion collector positioned in saidsmaller ionizationchamber, means for. maintaining a potentialdifference" between saidsecond electrode and said second ion collector so that said second ioncollector collects positive ions,sai d second electrodebeingmaintainedpositive with respect to said first collector so as. toprevent migration ofpositive ions into said smaller ionization.-chamber, asource of ionizing agents comprising-radium positioned toradiate alpha particles into saidtwoionizationchambersi, said sec: ondelectrode beingsubstantially permeable to alpha particles, meansconductively" connected to said collectors for amplifying currentcreated by the positive ions collected at. either of saidcollectors, andmeans connected to. the output of the amplifying means for indicatingthe magnitude of said amplified currents and thus the gas pressurewithin said ionization chambers.

5, A gas, pressure, gauge operable at pressures aboye atmosphericpressure without damage thereto and comprising an electrode whichdefines an ionization chamber, an ion collector Within the ionizationchamber, means for maintaining said ion collector at a potential whichis negative with respect to said electrode, a grid within saidionization chamber, said grid defining a second and smaller ionizationchamber, a second ion collector posi-v tioned in said smaller ionizationchamber, means for maintaining a potential difference between said gridand said second ion collector so that said second ion collect collectspositive ions, a source of ionizing agents comprising radium positionedto radiate alpha particles into said two ionization chambers, said gridbeing-constructed so as, not to interfere with the majority of alphaparticlesemittcd from said source, and means for maintaining said gridat a potential substantially preventing passage of positive ions intosaid smaller ionization chamber, said radium being positioned so thatsome of the alpha par-. ticles emitted therefrom pass through saidsmaller ionization chamber and into said larger ionization chamber,means conductively connected to said collectors for amplifyingcurrentcreated by the positive ions collected at either of said collectors, andmeans connected to the output ofthe amplifying means for indicating themagnitude of said amplified currents and thus the gas pressure withinsaid ionization chamber.

6. Agas pressure gauge operable at pressures above atmosphericpressurewithout damage thereto and prising; an electrode which defines anionization chambeg, an ion; collector within the ionization chamber,meaps for maintaining saidion collector at a potential which is negativewith respect to said electrode, a second elect-rode defining a secondand smaller ionization chamber which communicates with said firstionization chamber, a sgee 0nd ion colleotorpositioned in said smallerionizati nchamber, means for maintaining a potential difference betweensaid second electrode and said second ion collecs tor so that saidsecond ion collector collects positive ions, at least aportion of saidsecond electrode comprising a grid-like member separating said twoionization cl' atn-v hers, said grid-like member being maintained at apotential which is positive with respect to said two collectorelectrodes, said grid-like member permitting equalization of pressure:within said two ionization chambers, asourceof ionizing agentscomprising radium positiouedto radiate alpha particles into said twoionizationch-ambers, means conductively connected to said collectons foramplifying current created by the positive ions collected at either ofsaid collectors, and means connected to theoutputof the amplifying meansfor indicating the magnitudelofsaid' amplified currents and thus the gaspressure within said ionization chambers.

7'. The pressure gauge of claim 1 wherein said means for amplifying.current created by collection of ions comprises .a resistance networkfor selectively feeding, to a single amplifier, equivalent voltagescorresponding.- to ionization currents of widely different amplitudesfrom the two collectors, said resistance network being arranged sotliat'at low gas pressures the relativelyv small ionization current fromthe first ion collector develops a'voltage across arelativoly'hig'hinput resistance while at high gas pressuresthe'relatively large ionization current from second ion collectordevelops an equivalent voltage across'a relatively low input resistance,and a voltage divider betweensaid'low input resistance and saidamplifier, at least part-of said voltage divider comprising saidinput-Ii a 8,- App us-for measuring the pressure of a gas by nizationproduced in the gas, saidapp-aratus c9mprlsmg.-.twoioncollectors andmeans conductive'ly conuect'ed to'said" collectors for amplifying ioncurrents greases collected by either of said collectors, said amplifyingmeans comprising a resistance network for selectively feeding, to asingle amplifier, equivalent voltages corresponding to ionizationcurrents of widely difierent arnplitudes from the two collectors, saidresistance network being arranged so that at low gas pressures therelatively small ionization current from the first ion collectordevelops a voltage across a relatively high input resistance while athigh gas pressures the relatively large ionization current from thesecond ion collector develops an equivalent voltage across a relativelylow input resistance, a voltage divider between said low inputresistance and said amplifier, at least part of said voltage dividercomprising said high input resistance, and means connected to the outputof the amplifier for indicating the magnitude of said amplified currentsand thus the gas pressure to be measured.

References Cited in the file of this patent UNITED STATES PATENTS

